UA makes a certain degree of sense. They have found a great location in Budapest for Going Postal, so an Ankh-Morpork story makes sense. Doing a Watch book would mean seriously committing to an actor to play Sam Vimes and that's not going to be easy as we saw with the furor over David Jason and Sean Astin as Rincewind and Twoflower. Vetinari is one thing - but he's only a relatively small part. The casting for Vimes (and Granny if they ever do a Witch book) is going to be crucial and probably controversial.
UA shouldn't have those problems.
“Men never commit evil so fully and joyfully as when they do it for religious convictions.” – Blaise Pascal